Contact bonding adhesives

ABSTRACT

Pressure-sensitive adhesives comprising an aqueous polymer dispersions wherein the polymer is composed of 
     from 50 to 99.9% by weight of at least one C 1 -C 12  alkyl (meth)acrylate (monomers a) 
     from 0.05 to 20% by weight of at least one vinylaromatic compound (monomers b) 
     from 0.05 to 10% by weight of at least one ethylenically unsaturated hydroxy compound (monomers c) 
     from 0 to 10% by weight of an ethylenically unsaturated acid or an acid anhydride (monomers d) and 
     from 0 to 30% by weight of other ethylenically unsaturated compounds (monomers e) 
     the weight percentages being based on the polymer, and the polymer dispersion comprising an emulsifier consisting to the extent of at least 5% by weight of aromatic carbon atoms (and called aromatic emulsifier for short) or an emulsifier mixture consisting to the extent of at least 10% by weight of an aromatic emulsifier.

The present invention relates to pressure-sensitive adhesives comprisingan aqueous polymer dispersion, wherein the polymer is composed of

from 50 to 99.9% by weight of at least one C₁-C₁₂ alkyl (meth)acrylate(monomers a)

from 0.05 to 20% by weight of at least one vinylaromatic compound(monomers b)

from 0.05 to 10% by weight of at least one ethylenically unsaturatedhydroxy compound (monomers c)

from 0 to 10% by weight of an ethylenically unsaturated acid or an acidanhydride (monomers d) and

from 0 to 30% by weight of other ethylenically unsaturated compounds(monomers e)

the weight percentages being based on the polymer, and the polymerdispersion comprising an emulsifier consisting to the extent of at least5% by weight of aromatic carbon atoms (and called aromatic emulsifierfor short) or an emulsifier mixture consisting to the extent of at least10% by weight of an aromatic emulsifier.

Pressure-sensitive adhesives (PSAs) form a permanently tacky film whichadheres to a very wide variety of surfaces even under slight pressure atroom temperature. Pressure-sensitive adhesives are used to produceself-adhesive products such as labels, tapes and films. Products of thiskind are very easy to use and make it possible to work rapidly whenbonding. In contrast to contact adhesive compositions, no ventilationtimes are required. Moreover, there is no “open time” within which theadhesive bond must be made. The quality of a self-adhesive articledepends essentially on whether the cohesion (internal strength of thefilm of adhesive) and its adhesion (to the surface that is to be bonded)are in tune with one another in accordance with the application.

In the case of pressure-sensitive adhesives for sheets, adhesive tapesor labels, in particular, the level of cohesion must be sufficient forno stringing and no emergence of glue at the edges to occur in thecourse of stamping and cutting, since otherwise the cutting tools becomesoiled and the cut faces sticky. At the same time, the adhesion shouldbe at a high level in order to provide good sticking on the substratethat is to be bonded.

In general, it is impossible to optimize adhesion and cohesionindependently of one another. There is a desire for measures whicheither lift the level of both properties, or at least maintain oneproperty unchanged while improving the other.

Adhesives, including pressure-sensitive adhesives, based on polyacrylatedispersions are known, for example, from WO 98/06763, WO 98/23656 orEP-A-625 557. Copolymers of this kind still do not have the desiredlevel of adhesion and cohesion or the desired ratio of the one to theother. DE 19818394 (OZ48974) discloses pressure-sensitive adhesives withan aromatic emulsifier.

It is an object of the present invention to provide pressure-sensitiveadhesives having improved adhesion and/or cohesion.

We have found that this object is achieved by the pressure-sensitiveadhesives defined above.

The polymer of the aqueous polymer dispersion is composed of themonomers a) to e) defined at the outset.

The monomers a) comprise a C₁-C₁₂ alkyl (meth)acrylate, including inparticular mixtures of the alkyl (meth)acrylates.

Suitable preferably are C₁-C₈-alkyl (meth)acrylates.

Suitable examples are methyl acrylate, methyl methacrylate, n-butylacrylate, n-butyl methacrylate, 2-ethylhexyl acrylate and ethylacrylate.

Monomers b) comprise preferably α-methylstyrene or styrene.

Particular preference is given to styrene. The proportion of monomers b)in the polymer is preferably not more than 10%, in particular not morethan 5% and, with particular preference, not more than 3% by weight, andis preferably more than 0.1% by weight, in particular more than 0.5.

Monomers c) are preferably C₁-C₁₈ hydroxyalkyl (meth)acrylates.

Particular preference is given to C₂ to C₈ hydroxyalkyl (meth)acrylates.Suitable examples are hydroxypropyl acrylate and methacrylate, andhydroxy-n-butyl acrylate and methacrylate.

Hydroxypropyl acrylate or hydroxypropyl methacrylate is particularlypreferred.

The proportion of the monomers c) in the polymer is preferably up to10%, in particular 5% and, with particular preference, 3% by weight,preferably at least 0.1% by weight and, in particular, at least 0.5% byweight.

Monomers d) are, for example, ethylenically unsaturated monomers havingin particular carboxyl groups, such as (meth)acrylic acid, maleic acid,ethylenically unsaturated acid anhydrides or monoesters, such as maleicanhydride or maleic or fumaric monoesters. Acrylic and methacrylic acidare preferred. The proportion of monomers d) is preferably not more than5% by weight, based on the polymer.

Further monomers, e), can be of any kind. Preferred examples which maybe mentioned are vinyl esters, olefins, diolefins, ethylenicallyunsaturated amides or nitriles etc.

The polymer consists preferably of

from 60 to 99.5 % by weight of monomers a) from 0.05 to 5 % by weight ofmonomers b) from 0.05 to 5 % by weight of monomers c) from 0 to 5 % byweight of monomers d) from 0 to 25 % by weight of monomers e)

and with particular preference of

from 75 to 99.8 % by weight of monomers a) from 0.1 to 3 % by weight ofmonomers b) from 0.1 to 3 % by weight of monomers c) from 0 to 5 % byweight of monomers d) from 0 to 15 % by weight of monomers e)

The glass transition temperature of the polymer can be determined bycustomary methods, such as differential thermal analysis or differentialscanning calorimetry (see e.g. ASTM 3418/82, midpoint temperature).

The glass transition temperature (Tg) of the polymer is preferably from−60° C. to +10° C., more preferably from −50° C. to −10° C. and, withvery particular preference, from −50° C. to −20° C.

The polymer is preferably prepared by emulsion polymerization and istherefore an emulsion polymer.

In emulsion polymerization it is common to use ionic and/or nonionicemulsifiers and/or protective colloids, or stabilizers, assurface-active compounds.

In the present case use is made in accordance with the invention of anemulsifier consisting to the extent of at least 5% by weight of aromaticcarbon atoms (called aromatic emulsifier for short) or an emulsifiermixture which comprises at least 10% by weight of such an aromaticemulsifier, based on the overall amount of emulsifier.

The aromatic emulsifier preferably consists to the extent of at least10% by weight, with particular preference at least 20% by weight, ofaromatic carbon atoms (by which are meant carbon atoms that are part ofan aromatic ring system).

The proportion of aromatic carbon atoms is generally below 80% byweight.

The aromatic emulsifier is preferably an ionic emulsifier, especiallyone having one or two, preferably two, sulfate groups.

The molar weight of the emulsifier is preferably less than 2000 andespecially less than 1000 g/mol.

With particular preference, the aromatic emulsifier comprises an ioniccompound having a molecular weight of less than 2000 g/mol andcomprising at least one sulfate group or sulfonate groups, preferablytwo sulfate groups or sulfonate groups with two unsubstituted orsubstituted phenyl groups.

Preference is given to aromatic emulsifiers having one or two,preferably two, sulfonate groups.

The emulsifier particularly preferably comprises a compound of theformula

where X is O, S, CH₂, NH or NR⁷, one or two of R¹ to R⁶ are SO₃^(⊖)K^(⊕)and the others of R¹ to R⁶ are H or C₁-C₁₈ alkyl, R⁷ is C₁-C₁₈alkyl and K is a countercation.

X is preferably O. Preferably, one or two of R¹ to R⁶ are C₁-C₁₈ alkyl,especially C₆-C₁₈ alkyl, and the others of R¹ to R⁶ are hydrogen atomsand the sulfonate groups.

K is preferably a countercation selected, for example, from the alkalimetals, ammonium and hydrogen. Sodium is particularly preferred.Compounds of the formula I normally also comprise a mixture of compoundshaving different degrees of substitution (mono- or dialkylated) anddifferent positions of the substituents (of the sulfonate groups and ofthe one or two alkyl groups). Compounds having the formula I aremarketed under the tradename Dowfax® 2A by Dow Chemical Company.

Relative to the use of an aromatic emulsifier alone, the use of amixture comprising an aromatic emulsifier and an emulsifier withoutaromatic carbon atoms (nonaromatic emulsifier) is preferred.

The nonaromatic emulsifier is preferably of the formula

R⁸—O—(Z—O)_(n)—SO⁻ ₃K⁺  II

where:

R⁸ is C₁-C₁₈ alkyl, preferably C₁₀-C₁₆

Z is CH₂—CH₂ or

preferably CH₂—CH₂

n is an integer from 1 to 40, preferably from 2 to 30

K⁺ is a cation, e.g. H⁺, an alkali metal cation of e.g. Na or K, orammonium.

Compounds of the formula II are intended to embrace compounds comprisingboth

Compounds of the formula II are marketed, for example, under thedesignation Disponil® FES 77 by Henkel.

The emulsifier mixture consists preferably of

from 20 to 95% by weight of an aromatic emulsifier, especially that ofthe formula I, and

from 5 to 80% by weight of a nonaromatic emulsifier, especially that ofthe formula II.

The proportion of the aromatic emulsifier is preferably from 30 to 95%by weight and that of the nonaromatic emulsifier from 5 to 70% byweight, the weight percentages being based on the overall amount of theemulsifiers.

The emulsifier or emulsifier mixture is normally used in amounts of from0.1 to 15% by weight, preferably from 0.3 to 5% by weight and, withparticular preference, from 0.5 to 3% by weight, based on the monomersto be polymerized. Accordingly, the resulting polymer dispersionincludes the stated amounts of emulsifier.

Examples of water-soluble initiators for the emulsion polymerization areammonium salts and alkali metal salts of peroxodisulfuric acid, such assodium peroxodisulfate, hydrogen peroxide, or organic peroxides, such astert-butyl hydroperoxide.

Particular suitability is possessed by what are known asreduction/oxidation (redox) initiator systems.

The redox initiator systems consist of at least one reducing agent,which is usually inorganic, and one organic or inorganic oxidizingagent.

The oxidizing component comprises, for example, the abovementionedemulsion polymerization initiators.

Reducing components comprise, for example, alkali metal salts ofsulfurous acid, such as sodium sulfite, sodium hydrogen sulfite, alkalimetal salts of disulfurous acid, such as sodium disulfite, bisulfiteadducts with aliphatic aldehydes and ketones, such as acetone bisulfite,or reducing agents such as hydroxymethanesulfinic acid and its salts, orascorbic acid. The redox initiator systems can be used along withsoluble metal compounds whose metallic component is able to exist in aplurality of valence states.

Examples of customary redox initiator systems are ascorbic acid/iron(II) sulfate/sodium peroxodisulfate, tert-butyl hydroperoxide/sodiumdisulfite, tert-butyl hydroperoxide/sodium hydroxymethanesulfinate. Theindividual components, such as the reducing component, can also comprisemixtures; for example, a mixture of sodium hydroxymethanesulfinate andsodium disulfite.

Said compounds are generally employed in the form of aqueous solutions,the lower concentration being determined by the amount of water which isacceptable in the dispersion and the upper concentration by thesolubility of the relevant compound in water. The concentration isgenerally from 0.1 to 30% by weight, preferably from 0.5 to 20% byweight and, with particular preference, from 1.0 to 10% by weight, basedon the solution.

The amount of the initiators is generally from 0.1 to 10% by weight,preferably from 0.5 to 5% by weight, based on the monomers to bepolymerized. It is also possible to use two or more different initiatorsin the course of the emulsion polymerization.

Regulators as well can also be employed for the polymerization, andreduce the molecular mass. Examples of suitable compounds are thosehaving a thiol group, such as tert-butyl mercaptan, ethylhexylthioglycolate, mercaptoethanol, mercaptopropyl-trimethoxysilane ortert-dodecyl mercaptan. The proportion of these regulators can inparticular be from 0 to 0.3% by weight, preferably from 0.02 to 0.3% byweight, based on the polymer.

The emulsion polymerization is generally carried out at from 30 to 100°C., preferably from 50 to 95° C. The polymerization medium can consisteither of water alone or of mixtures of water with water-miscibleliquids such as methanol. Use is preferably made of water alone. Theemulsion polymerization can be conducted either as a batch process or inthe form of a feed process, including staged or gradient procedures.Preference is given to the feed process, in which a portion of thepolymerization mixture is introduced as initial charge, heated to thepolymerization temperature and initially polymerized, and then theremainder of the polymerization mixture is supplied to thepolymerization zone continuously, in stages or under a concentrationgradient and normally by way of two or more spatially separate feedstreams of which one or more comprise the monomers in pure or inemulsified form, this supply taking place with the polymerization beingmaintained.

The manner in which the initiator is added to the polymerization vesselin the course of free-radical aqueous emulsion polymerization is knownto the skilled worker. It can either be introduced entirely in theinitial charge to the polymerization vessel or else insertedcontinuously or in stages at the rate at which it is consumed in thecourse of the free-radical aqueous emulsion polymerization. In eachindividual case it will depend in a manner known to the skilled workerboth on the chemical nature of the initiator system and on thepolymerization temperature. It is preferred to introduce one portion inthe initial charge and to supply the remainder to the polymerizationzone at the rate at which it is consumed.

In order to remove the residual monomers it is also customary, followingthe end of the actual emulsion polymerization, i.e. after a monomerconversion of at least 95%, to add initiator.

In the case of the feed process, the individual components can be addedto the reactor from above, through the side or from below, through thereactor floor.

The emulsion polymerization gives aqueous dispersions of the polymer,generally having solids contents of from 15 to 75% by weight and,preferably, from 40 to 75% by weight.

For a high space/time yield of the reactor, dispersions having a veryhigh solids content are preferred. In order to be able to obtain solidscontents >60% by weight it is advisable to establish a bimodal orpolymodal particle size, since otherwise the viscosity becomes too highand the dispersion can no longer be handled. Producing a new generationof particles can be carried out, for example, by adding seed (EP 81083),by adding excess emulsifier, or by adding miniemulsions. A furtheradvantage associated with the low viscosity at high solids content isthe improved coating behavior at high solids contents. The production of(a) new particle generation(s) can be made at any desired point in time.It is guided by the target particle-size distribution for low viscosity.

The resulting polymers or polymer dispersions are used aspressure-sensitive adhesives. They are particularly suitable as PSAs forlabels or adhesive tapes.

The polymers are preferably used in the form of their aqueousdispersion. The aqueous polymer dispersions can be used aspressure-sensitive adhesives without further additives.

In the case of use as pressure-sensitive adhesive it is possible to addto the polymers or to the aqueous polymer dispersions a tackifier, i.e.a tackifying resin. Tackifiers are known, for example, from AdhesivesAge, July 1987, pages 19-23 or Polym. Mater. Sci. Eng. 61 (1989), pages588-592.

Examples of tackifiers are natural resins, such as rosins and theirderivatives by disproportionation or isomerization, polymerization,dimerization or hydrogenation. They can be in their salt form (with, forexample, mono- or polyvalent counterions (cations)) or, preferably, intheir esterified form. Alcohols used for esterification can be mono- orpolyhydric. Examples are methanol, ethanediol, diethylene glycol,triethylene glycol, 1,2,3-propanethiol and pentaerythritol.

Also used, furthermore, are hydrocarbon resins, examples beingindene-coumarone resins, polyterpene resins, hydrocarbon resins based onunsaturated CH compounds, such as butadiene, pentene, methylbutene,isoprene, piperylene, divinylmethane, pentadiene, cyclopentene,cyclopentadiene, cyclohexadiene, styrene, α-methylstyrene, vinyltoluene.

Further compounds increasingly being used as tackifiers arepolyacrylates of low molecular weight. These polyacrylates preferablyhave a weight-average molecular weight M_(W) of less than 30,000. Thepolyacrylates consist preferably to the extent of at least 60% byweight, in particular at least 80% by weight, of C₁-C₈ alkyl(meth)acrylates.

Preferred tackifiers are naturally occurring or chemically modifiedrosins. Rosins consist predominantly of abietic acid or its derivatives.

The tackifiers can be added in a simple manner to the polymers of theinvention, preferably to the aqueous dispersions of the polymers. Thetackifiers are preferably themselves in the form of an aqueousdispersion in this case.

The proportion of the tackifiers is preferably from 5 to 100 parts byweight, with particular preference 10 to 50 parts by weight, based on100 parts by weight of polymer (solids/solids).

In addition to tackifiers it is also possible, for example, for otheradditives, such as thickeners, antifoams, plasticizers, pigments,wetting agents or fillers, to be employed in the case of utility as apressure-sensitive adhesive. The pressure-sensitive adhesives of theinvention may therefore also comprise not only the aqueous polymerdispersion but also tackifiers and/or the above additives.

The pressure-sensitive adhesives can be applied by customary methods,such as by rolling, knifecoating, brushing, etc., to substrates,examples being paper or polymer films consisting preferably ofpolyethylene, polypropylene, which can be biaxially or monoaxiallyoriented, polyethylene terephthalate, polyvinyl chloride, polystyrene,polyamide or metal. The water can be removed preferably by drying atfrom 50 to 150° C. For subsequent use, the PSA-coated side of thesubstrates, e.g. the labels, can be covered with a release paper, suchas with a siliconized paper.

The pressure-sensitive adhesives of the invention exhibit 30 improvedadhesion and cohesion and/or an improved adhesion/cohesion ratio.

EXAMPLES A) Test Methods

a) Preparing the Test Strips

The test dispersion is applied to siliconized paper in a thin film,using a coating bar, and is dried at 90° C. for 3 minutes. The gapheight of the coating bar is chosen so that the weight per unit area ofthe dried adhesives is 19-21 g/m². A commercially customary OPP film(30μ, corona-pretreated) is placed on the dried adhesive and rolled onfirmly using a manual roller. The resulting film laminate is cut into0.5 inch strips 2.5 cm wide. Prior to testing, these strips are storedunder standard atmospheric conditions for at least 24 h.

b) Testing the Shear Strength as a Measure of the Cohesion (inAccordance With FINAT FTM 7)

After the siliconized paper has been peeled off, the film test strip isbonded to the edge of a stainless steel sheet so as to give a bond areaof 2.5 inch×2.5 cm. 10 minutes after bonding, a 1000 g weight isfastened to the protruding end of the film strip, and the metal sheet issuspended vertically. Ambient conditions: 23° C., 50% relative humidity.The shear strength is taken as the time, in minutes, until the adhesivebond fails under the influence of the weight, as the mean from theresults of three test specimens.

c) Testing the Peel Strength as a Measure of the Adhesion (in AccordanceWith FINAT FTM 1)

After the siliconized paper has been peeled off, a film strip 2.5 cmwide is bonded to a stainless steel sheet. Ambient conditions: 23° C.,50% relative humidity. 1 minute after bonding (AFERA substrate) or 24hours after bonding (polyethylene substrate), the strip is peeled off atan angle of 180° and at a rate of 300 mm/min with the aid of a tensiletesting apparatus. The peel strength is taken as the force, in N/2cm,required to do this on average, as a mean from the results of three testspecimens.

B) Preparing the Dispersions Comparative Example V1

In a polymerization reactor, a solution of 0.76 g of ascorbic acid in200 g of water is heated to 90° C. with stirring and then, whilestirring continues, an aqueous solution of 20.27 g of sodiumperoxodisulfate (concentration 7% by weight) is added. After 5 minutes,feed stream 1 is started and is metered over the first 6 minutes at arate of 1.3 g/min. For a subsequent 13 minutes, the rate of addition offeed stream 1 is raised to 1.4 g/min, for a further 30 minutes to 2.9g/min and then for a further 13 minutes to 4.3 g/min. After that point,the rate of addition is raised to 7.73 g/min and held constant over afurther 225 min. Concurrently with feed stream 1, the addition iscommenced of a solution of 5.16 g of sodium peroxodisulfate and 68 g ofwater, which is metered in at a constant rate over 270 minutes.Following the end of feed stream 1, 15 g of a 10% strength solution oftert-butyl hydroperoxide in water is metered concurrently with asolution of 1.5 g of sodium disulfite in a mixture of 34.3 g of waterand 0.9 g of acetone, at 90° C. and with stirring. Finally, at 90° C.,15 g of a 50% strength aqueous solution of a dioctyl sulfosuccinate areadded over 15 minutes. The solids content of the dispersions is adjustedto 68-70%.

Feed Stream 1

287 g of water

50 g of a 30% strength aqueous solution of the sodium salt of thesulfuric monoester of dodecanol ethoxylated with 30 ethylene oxide units(Disponil FES77)

6.7 g of a 45% strength aqueous solution of the sodium salt of diphenylether derivatized with one C₁₂-C₁₄ alkyl radical and with two sulfonylradicals (Dowfax 2A1)

30 g of a 10% strength aqueous solution of sodium hydroxide

7.5 g of acrylic acid AA (0.5%)

165 g of methyl methacyrlate MMA (11%)

1327.5 g of 2-ethylhexyl acrylate EHA (88.5%)

For the other examples indicated, the procedure of Comparative Example 1was repeated with the difference that in feed stream 1 the monomeramounts indicated in the Table below were used.

The following abbreviations have been used:

V: Comparative Examples

E: Examples in accordance with the invention

EHA: 2-Ethylhexyl acrylate

BA: n-Butyl acrylate

VAc: Vinyl acetate

MMA: Methyl methacrylate

AA: Acrylic acid

S: Styrene

HPA: Hydroxypropyl acrylate

S (Afera): Peel strength on Afera, immediate

S (PE): Peel strength on polyethylene after 24 h

K: Cohesion in [min] on Afera

pphm: Percent based on monomers

Emulsifier a: 1.0 pphm Disponil FES77, 0.2 pphm Dowfax 2A1

Emulsifier b: 1.2 pphm Disponil FES77

Emulsifier c: 1.2 pphm Steinapol NLS (45% strength aqueous solution ofsodium lauryl sulfate)

Emulsifier d: 1.2 pphm Texapon NSO-IS (28% strength aqueous solution ofthe sodium salt of the sulfuric monoester of dodecanol ethoxylated with2-3 ethylene oxide units)

TABLE 1 No. Emulsifier EHA BA VAc MMA AA S HPA S (Afera) S (PE) K Tg V1aa 88.5 11  0.5 — — 3.7 4.3 1553 −49 V2 a 88.5 10  0.5 1 — 3.9 3.9 2013−48.9 V3 a 88.5 9 0.5 2 — 3.8 4.4 2832 −49 V4 a 88.5 6 0.5 5 — 4.8 13.3868 −53 V5 a 87.5 11  0.5 — 1 3.9 3.9 843 −50 V6 a 86.5 11  0.5 — 2 3.95 1242 −48 V7 a 83.5 11  0.5 — 5 3.9 4.5 1728 −45 E1 a 87.5 10  0.5 1 14 5 2629 −48.3 E2 a 86.5 9 0.5 2 2 4.7 5.3 5333 −49.7 E3 a 83.5 6 0.5 55 4.2 8.4 595 −45.9 E4 a — 86.5 9 0.5 2 2 3.3 2.8 >7200 −31 E5 a 40  46.5 9 0.5 2 2 3.8 3.9 >7200 −39.8 E6 a 86.5 — 0.5 11  2 6.4 11.2 61 −49V8 a 81.5 8 8 0.5 — — 3.8 4.7 >7200 −44 E7 a 79.5 8 10  0.5 2 2 4.65.5 >7200 −42 V9 a 99.5 — 0.5 — — 1.6 2.5 93 −63 V10 a 97.5 — 0.5 2 —2.5 4.1 373 −62.6 V11 a 97.5 — 0.5 — 2 1.7 2.7 186 −61 E8 a 95.5 — 0.5 22 2.8 4.9 621 −59.9 V1b b 88.5 11  0.5 — — 3.7 4.5 1071 V1c c 88.5 11 0.5 — — 3.4 3.7 1181 V1d d 88.5 11  0.5 — — 3.9 4.2 1436 V(E1)b b 87.510  0.5 1 1 5.7 — 1504 V(E1)c c 87.5 10  0.5 1 1 5 4.9 2680 V(E1)d d87.5 10  0.5 1 1 4.8 3.7 1198 V(E2)b b 86.5 9 0.5 2 2 6.6 7 1566 V(E2)cc 86.5 9 0.5 2 2 4.7 3.5 415 V(E2)d d 86.5 9 0.5 2 2 5.7 — 307 V(E4)b b— 86.5 9 0.5 2 2 4.1 4.5 5770 V(E4)c c — 86.5 9 0.5 2 2 3.1 4.8 1916V(E4)d d — 86.5 9 0.5 2 2 2.6 5.5 1474 V(E5)b b 40   46.5 9 0.5 2 2 5.46.7 4128 V(E5)c c 40   46.5 9 0.5 2 2 4.7 6.5 1652 V(E5)d d 40   46.5 90.5 2 2 4.9 4.8 1102

We claim:
 1. A pressure-sensitive adhesive comprising an aqueous polymerdispersion, wherein said polymer comprises from 50 to 99.9% by weight ofat least one C₁-C₁₂ alkyl (meth) acrylate monomer a), from 0.05 to 20%by weight of at least one vinylaromatic compound monomer b), from 0.05to 10% by weight of at least one ethylenically unsaturated hydroxycompound monomer c), from 0 to 10% by weight of an ethylenicallyunsaturated acid or an acid anhydride monomer d), from 0 to 30% byweight of at least one other ethylenically unsaturated compound monomere), the weight percentage based on the polymer, and said polymerdispersion comprises an emulsifier mixture, wherein said emulsifiercomprises from 20 to 95% by weight of an aromatic emulsifier and from 5to 80% by weight of a sulfate of the formula (II) R⁸—O—(Z—O)_(n)—SO₃⁻K⁺  (II)  where: R⁸ is C₁-C₁₈ alkyl,

n is an integer from 1 to 40, K is a cation, and the aromatic emulsifieris an emulsifier which comprises at least 5% by weight of aromaticcarbon atoms.
 2. The adhesive as claimed in claim 1, wherein the polymercomprises from 60 to 99.9% by weight of monomer a), from 0.05 to 5% byweight of monomer b), from 0.05 to 5% by weight of monomer c), from 0 to5% by weight of monomer d), and from 0 to 25% by weight of monomer e).3. The adhesive as claimed in claim 1, wherein monomer c) comprises atleast one C₁-C₈ hydroxyalkyl (meth)acrylate.
 4. The adhesive as claimedin claim 1, wherein monomer b) comprises styrene.
 5. The adhesive asclaimed in claim 1, wherein monomer a) comprises at least one C₁ to C₈alkyl (meth)acrylate.
 6. The adhesive as claimed in claim 1, wherein thearomatic emulsifier comprises an ionic compound having a molecularweight of less than 1000 g/mol and at least one sulfate group orsulfonate group.
 7. The adhesive as claimed in claim 6, wherein thearomatic emulsifier comprises a compound of the formula (I)

where: X is O, S, CH₂, NH or NR⁷, one or two of R¹ to R⁶ are SO₃ ⁻K⁺ andthe others of R¹ to R⁶ are H or C₁-C₁₈ alkyl, R⁷ is C₁-C₈ alkyl, and Kis a countercation.
 8. A label, sheet or adhesive tape comprising asubstrate coated with the adhesive as claimed in claim
 1. 9. An aqueouspolymer dispersion comprising a polymer wherein said polymer comprisesfrom 50 to 99.9% by weight of monomer a), from 0.05 to 20% by weight ofmonomer b), from 0.05 to 10% by weight of monomer c), from 0 to 10% byweight of monomer d), from 0 to 30% by weight of monomer e), and theemulsifier mixture as claimed in claim 1.